RFR: 8304301: Remove the global option SuperWordMaxVectorSize

[Fei Gao]

On Wed, 22 Mar 2023 21:58:09 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

> I am not sure about this effect of UseAVX setting on command line. We use UseAVX setting on command line to emulate runs on different platforms. Some our machines are Cascade Lake. Before, even if we use `-XX:UseAVX=3` on command line we still limit superword vector size for CL. With this patch we will not do that. It will not match what happens in production with default setting and we miss testing coverage for CL.
> 
> Why you added `FLAG_IS_DEFAULT(UseAVX)` check?

Hi @vnkozlov, thanks for your review!

I added the `FLAG_IS_DEFAULT(UseAVX)` check to follow its original logic, see https://github.com/openjdk/jdk/blob/91f407d6fe285c44bcc25c1acdf5dc0c43be0172/src/hotspot/cpu/x86/vm_version_x86.cpp#L1317. Before the patch, using `UseAVX=3` alone would change superword vector size for CL to be `MaxVectorSize`, but we could still explicitly use `-XX:SuperWordMaxVectorSize=32` to limit superword vector size for CL.

Certainly, removing `FLAG_IS_DEFAULT(UseAVX)` check here is also reasonable to me. Then only `-XX:MaxVectorSize=64` will affect it.

-------------

PR Comment: https://git.openjdk.org/jdk/pull/13112#issuecomment-1480477134